SQL, update a column with values from another table

I'm always forgetting and re-learning this one. Why won't it stick? Now it's here as an aide-memoire.

The scenario is: you have an empty column in one table which you want to update with related values in another table. How to choose the values? Well, in my case, they are dependent on a different column in the first table. Typically, I sometimes find myself importing a table from a sheet, and then starting to normalise it by re-presenting some columns with numbered IDs instead of their original (inefficient) strings. So temporarily the string remains in the first table, side by side with the column which will be the normalised ID (foreign key into the second table).